1. Field of the Invention
The present invention relates to a scanning optical apparatus, and more particularly to a scanning optical apparatus suitably applicable, for example, to apparatus such as laser beam printers (LBP) or digital copiers including the electrophotographic process in which an optically modulated beam (light beam) emitted from light source means is deflected and reflected by a deflecting device (optical deflector) consisting of a rotary polygon mirror etc. and thereafter a scanned surface is optically scanned therewith through an imaging device (f.theta. lens) having the f.theta. characteristics to record image information.
2. Related Background Art
In conventional scanning optical apparatus such as laser beam printers the light source means emits the optically modulated beam according to an image signal. Then the optically modulated beam is periodically deflected by the optical deflector, for example, consisting of the rotary polygon mirror, thereafter the beam is converged in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system having the f.theta. characteristics, and the surface is optically scanned with the beam to effect image recording.
FIG. 1 is a schematic drawing to show major part of a conventional scanning optical apparatus.
In the drawing a divergent beam emitted from the light source means 51 is converted into a nearly parallel beam by a collimator lens 52, a stop 53 limits the beam (quantity of light), and the thus shaped beam is incident into a cylindrical lens 54 having a predetermined refractive power only in a sub-scan direction normal to the plane of the drawing. Among the parallel beam incident into the cylindrical lens 54, the beam is emergent in a state of parallel beam as it is in a cross section along the main scan direction parallel to the plane of drawing. In contrast, the beam is converged in a cross section of sub scan to form a nearly linear image on a deflection/reflection facet 55a of the optical deflector 55 consisting of the rotary polygon mirror.
Then the beam deflected and reflected by the deflection facet 55a of the optical deflector 55 is guided through an imaging optical system (f.theta. lens) 56 onto a surface of photosensitive drum 58 as a scanned surface, and the surface of the photosensitive drum 58 is optically scanned with the beam in the main scan direction by rotating the optical deflector 55 in the direction of arrow A, thereby recording image information.
In order to adjust a scan start position on the surface of photosensitive drum 58, prior to the optical scan on the surface of photosensitive drum 58, part of the beam deflected and reflected by each deflection facet of the optical deflector 55 is let to pass a region before a region where the beam for scanning the surface of photosensitive drum 58 passes and then is reflected by a return mirror (BD mirror) 57 to be guided to a detection device (BD sensor) 59 for obtaining a scan start signal (or for obtaining a synchronizing signal of image writing start position). Then utilizing an output signal from the detection device 59 (detection signal of image writing start position (BD signal)), the scan start position of image recording onto the surface of photosensitive drum 58 is adjusted.
In the conventional scanning optical apparatus the part of beam deflected and reflected by the optical deflector 55 is guided through the f.theta. lens 56 to the BD sensor 59 in order to obtain the detection signal of image writing start position, i. e, the so-called BD signal.
The scanning optical apparatus of such a structure necessitated that a distance from the f.theta. lens 56 to the BD sensor 59 was made equal to the focal length of the f.theta. lens 56, so that the entire apparatus was likely to become large in size.
In the case of the conventional scanning optical apparatus, the fixed BD mirror 57 is provided as a part of an optical system (synchronous detection system) for obtaining the scan start signal to the scanned surface 58, as shown in FIG. 1, so that the BD mirror 57 properly bends an optical path of the beam (BD beam) for detection signal of image writing start position (synchronizing signal) passing through the f.theta. lens 56, thereby permitting the entire apparatus to be formed in a compact shape.
Such scanning optical apparatus, however, had problems: the adjustment of writing start position must be performed precisely because the image writing start position could deviate depending upon mounting accuracy of the BD mirror 57; an optically effective region in the main scan direction, of the f.theta. lens 56 was not allowed to be expanded because the BD beam was focused using the lens region (the region other than the region where the scanning beam passes) of a part of f.theta. lens 56, etc. Thus, these problems made difficult compatibility of size reduction and cost reduction of the scanning optical apparatus.